Write checker for new docs schema

[mark-wiemer]

Once #297 is done, we should write a checker program that takes in a Markdown document and returns whether the document matches the schema. If it doesn't match, the program should return detailed error messages.

Ideally, we can easily extend this program to check just snippets of a document instead of the whole thing. It would take in a different schema as well to check against.

[mark-wiemer]

Getting ahead of myself a bit, but finally found time to write some code :)

https://github.com/mark-wiemer/hello-hello/tree/261d0565aa4ea3258f57cfc21a2efbf4d9e49fdc/packages/luanti/doc-schema

• Takes in lua_api.md
• Checks whether it has exactly 1 h1 element
• Checks whether that h1 element has the exact text "Luanti Lua Modding API Reference"
• Logs warnings if any checks fail
• Includes comprehensive unit tests

Basically, this lays the groundwork for future checks, like making sure each function definition documents all of its args, etc. Still need to work on #297 first, of course, but this was more fun :)